The existing placeholder can make the user think that they must use http. We should encourage the use of https.
Also, the block editor uses https for its placeholders. Let's add some consistency.
Props Chaton666, audrasjb, mukesh27.
Fixes #46320.
git-svn-id: https://develop.svn.wordpress.org/trunk@45777 602fd350-edb4-49c9-b593-d223f7449a82